This invention relates to an apparatus for controlling the operation of a water turbine or a pump turbine and a method thereof.
During the power generation operation, a "water turbine or a pump turbine" is controlled by a speed governor. The speed governor detects the rotation speed of the water turbine or the pump turbine, opens and closes guide vanes according to a deviation with respect to a predetermined rotation speed, adjusts water energy supplied to the water turbine or the pump turbine, and keeps the rotation speed of the water turbine or the pump turbine to a predetermined value. The frequency of an electric power system is maintained constant by the action of the speed governor. Where any failure occurs in the electric power system, the governor acts as a protector for the water turbine or the pump turbine. However, in the case when lightning strikes the electric power system, the load of the generator is momentarily interrupted (hereinafter referred to as load rejection), so that water energy is no longer absorbed as electric power into the electric power system through the generator. For this reason, the water energy is instead converted to an acceleration energy to accelerate the water turbine or the pump turbine and thus cause the rotation speed of the water turbine or the pump turbine to increase sharply. In this case, the governor detects an increase in the rotation speed of the water turbine or the pump turbine, immediately closing the guide vanes etc. and decreasing the energy of water flowing into the water turbine or the pump turbine. In this way, the governor serves as a protector. Even if the rotation speed of the water turbine temporarily becomes higher than a rated rotation speed, the rotation speed of the water turbine or the pump turbine is reduce to the rated rotation speed or the predetermined rotation speed after lapse of a certain period. As a result, there is no possibility that the water turbine or the pump turbine will be permitted to rotate for a long period at a rotation speed higher than the rated rotation speed. It may, therefore, be said that the speed governor is a very important protector for the water turbine or the pump turbine during the power generation operation. For this reason, the speed governor requires not only a quick response but also absolute reliability with respect to an overall control system including the guide vanes and must have a high reliability in practice. In the water turbine or the pump turbine during the power generation operation there may rarely be encountered such an abnormal situation that in spite of detecting an increase in the rotation speed by the speed governor upon load rejection, the guide vanes are not closed due to the faults in, for example, a hydraulic system for opening and closing the guide vanes. In controlling the operation of the water turbine or the pump turbine it is important that a fail-safe protector be incorporated to prevent the propagation, over the whole power station, of a failure due to the guide vanes being not closed. An overspeed relay has been used as a protector for the case where the guide vanes are not closed upon load rejection. The overspeed relay is so constructed that it is operated when the rotation speed of the water turbine or the pump turbine becomes greater than a prescribed rotation speed N.sub.R in excess of a predetermined rated rotation speed. By the operation of the overspeed relay an emergency shutdown or a quick shutdown order is sent to a main machine. In this case, the emergency shutdown or the quick shutdown order is normally sent as a close order to both the inlet valve of the water turbine or the pump turbine and the guide vanes. Even if the guide vanes are not closed, for example, due to the failure of the hydraulic system, the water turbine or the pump turbine is protected, because the inlet valve is closed. In this method, however, the prescribed rotation speed N.sub.R at which the overspeed relay is operated cannot be set to a value near to the rated rotation speed. This is because if the prescribed rotation speed N.sub.R is set to a value near to the rated rotation speed the overspeed relay may be operated due to a slight decrease in the generator load even in the absence of any failure of the hydraulic system etc. The prescribed rotation speed N.sub.R at which the overspeed relay is operated is set to a rotation speed so high as to not usually encountered. Under these circumstances, when the guide vanes are not closed upon load rejection, the inlet valve is not immediately closed to permit the water turbine or the pump turbine to be promptly protected. This will be described in more detail.
Now suppose that with the generator operated with a power output of 100%, the rotation speed of the water turbine or the pump turbine reaches about 120 to 140% of the rated rotation speed upon momentary load rejection even if the speed governor or guide vanes perform a normal operation. Where the guide vanes are not closed, the water turbine or the pump turbine reaches a maximum runaway speed greater than when the guide vanes are closed. For this reason, the prescribed rotation speed N.sub.R at which the overspeed relay is operated is usually set to about 105% of a maximum overspeed when the guide vanes are normally operated. With the prescribed rotation speed N.sub.R set to a smaller, the time taken for the inlet valve to be closed will be shortened, but if the generator load is slightly decreased due to a minor failure of the electric power system the emergency or the quick shutdown of the water turbine or the pump turbine is effected, separating the generator from the electric power system. This impairs the original fundamental function that, while adjusting the output of the water turbine by opening or closing the guide vanes according to a change of the load through the use of the speed governor, the power generation operation is performed in the hydraulic power station. Such a situation is undesirable in the management of the electric power system.
When the operation control of the water turbine or the pump turbine is effected with the overspeed relay as a protector, if the guide vanes of the water turbine or the pump turbine are not closed even in the case of load rejection, it takes a considerable time (usually 5 to 10 seconds) for the inlet valve to start to be closed. Even where the inlet valve is closed, the water turbine or the pump turbine will reach a rotation speed about 10 to 20% as high as the maximum overspeed when the speed governor and guide vanes perform a normal operation. In an ordinary Francis type water turbine, even such a situation poses no problem in view of the flow rate characteristic below-mentioned. That is, since in this case only an increase in the rotation speed of the water turbine is involved, even if the water turbine undergoes a high-speed rotation, no problem is presented if a design consideration is paid to the strength of the apparatus with an adequate safety factor imparted thereto. In a method using the overspeed relay as a protector in the Francis type pump turbine and in particular the pump turbine of a lower specific speed (as determined by the configuration of the pump turbine) for a high head in particular, even where the pump turbine is so designed as to impart a large safety factor and thus endure even a high-speed rotation, if the guide vanes are not closed, a fairly long time will be required until the inlet valve starts to be closed, and the pump turbine will not be protected in a stable way, the reasons of which will be set out below.
That is, FIG. 1 shows the flow rate characteristic curve of a Francis type pump turbine of low specific speed. In FIG. 1, the abscissa shows the number of rotations, n, per unit head, and the ordinate shows a flow rate, q, per unit head. The solid line La in FIG. 1 shows the flow rate characteristic curve when the opening of the guide vanes corresponds to a substantially 100% opening of the guide vanes. In FIG. 1,
n=N/.sqroot.H PA1 q=Q/.sqroot.H PA1 N=the number of rotations of the pump turbine PA1 Q=the flow rate of the pump turbine PA1 H=the net head of the pump turbine PA1 H: the net head with the inlet valve fully opened PA1 H': the net head with the smaller opening of the inlet valve PA1 Q: the flow rate PA1 .zeta.: loss coefficient of the inlet valve PA1 (a) a load rejection detector connected to receive the rotation speed for comparing the rotation speed of the water turbine or the pump turbine with a first prescribed value and for producing a load rejection signal when the rotation speed of the water turbine or the pump turbine becomes greater than the prescribed value, PA1 (b) a governor state detector for receiving an input signal and for judging whether or not a speed governor performs a normal operation based on the input signal to produce a judgement signal when the speed governor is judged as being normal, PA1 (c) inlet valve open order device connected to receive the judgement signal for producing an output signal when neither an emergency shutdown order nor a quick shutdown order is issued, PA1 (d) an inlet valve close order device connected to receive the load rejection signal and connected to receive the output signal for producing an inlet valve close order based on the load rejection signal and the output signal,
Now suppose that on a point A on a curve La in FIG. 1 the pump turbine is operated. In this case, if the guide vanes are not closed with the generator load interrupted, the rotation speed N of the pump turbine is increased and the operation point of the pump turbine is shifted from the point A through a point B to an operation point C at no load. At this time, the rotation speed N of the pump turbine reaches a peak. This value becomes greater than the maximum overspeed of the pump speed when the guide vanes are closed. Thereafter, the operation point is quickly shifted from the point C to a point D where the pump turbine serves as a reverse pump turbine i.e. the water flows in the pumping-up direction while the pump turbine is driven in such a direction as to drive the generator. Then, a deceleration torque is applied to the pump turbine, causing the operation point to be shifted to a point E and then to a point F where water flows into the pump turbine. At the point F, an accelerating torque is again applied to the pump turbine and the operating point is shifted again to the point D through the point C. In this way, the flow rate characteristic curve takes an S-shape. If in the Francis type pump turbine of low specific speed the guide vanes are not closed, the operation point is periodically shifted between the water turbine operation area and the reverse pump operation area with the no-load operation point as a center. A pressure at the inlet or outlet side of the pump turbine suffers a greater change in flow rate for a brief period due to a rapid change in the operation state by the pump turbine and is rapidly increased or decreased by a water hammer operation resulting from an inertia occuring in the water conduit. If the tailrace is particularly long, the pressure at the inlet or outlet side of the pump turbine involves a greater water pressure drop when the operation point is shifted to the point D at the reverse pump area due to the no-load operation. Where the pressure at the tailrace side of the pump turbine becomes lower than a saturated vapor pressure, a water column separation occurs, causing the occurrence of cavitation. The cavitation is immediately collapsed with a pressure increase, but there is a risk that the tailrace or the pump turbine will be destroyed due to an impulsive pressure increase upon recombination of the water column. In this way, a pressure drop occurs at the tailrace side owing to the S-shaped flow rate characteristic. With a branch conduit system, in particular, sharing a plurality of pump turbines, a prominent pressure drop occurs due to a mutual action between the pump turbines. That is, if with the generator load momentarily interrupted with respect to all the pump turbines, the guide vanes of one pump turbine are not closed, an overlap takes place between a pressure drop at the tailrace side resulting from the S-shaped flow rate characteristic of said one pump turbine and a pressure drop at the tailrace side resulting from the closure of the guide vanes of the other pump turbines. For this reason, the pressure at the tailrace side of the pump turbine whose guide vanes are not operated is dropped to a much lower level.
Where in the water conduit system the ratio of the number of branch water conduits shared by the pump turbines to the whole water conduit system is smaller with the water level located higher than the center position of the pump turbine i.e. with the deeper static draft head involved, there occurs a situation where the guide vanes of the pump turbine are not closed upon load rejection, leading to no water column separation even if a pressure drop occurs at the tailrace side. Where in such a situation the conventional overspeed relay is used as a protector, even if the guide vanes are not closed, the inlet valve is closed by the operation of the overspeed relay, permitting the pump turbine to be safely protected.
Recently, to lower civil engineering and construction costs in power stations, however, a plurality of pump turbines have been mounted with the static draft head set as low as possible with respect to the pump turbine. A meeting point of the branch conduits is then provided in the neighborhood of the tailrace of the pump turbine. One pressure conduit is accordingly disposed between the meeting point and the water/pump turbine to permit the use of only one tailrace. Now suppose that in such a power station the conventional overspeed relay is used as a protector. If in this case the guide vanes are not closed upon load rejection, the overspeed relay starts to be operated a little before the rotation speed of the pump turbine reaches its peak value, starting to close the inlet valve. It is therefore impossible to avoid a pressure drop at the tailrace immediately after the rotation speed of the pump turbine reaches its peak value, as well as to avoid a water column separation due to the pressure drop. If an attempt is made to safely protect the tailrace and pump turbine at the power station even if the guide vanes are not closed, the inlet valve needs to be closed immediately after the load is interrupted. Since the flow rate characteristic curve of the Francis type water turbine is then as indicated by the curve Lb in FIG. 1, there is no possibility that the operation point will enter into the reverse pump area through the no-load operation point C". Even in the pump turbine having such a characteristic as indicated by a curve La in FIG. 1, if the inlet valve is opened to a smaller extent, the characteristic of the pump turbine becomes that as indicated by a curve Lc. With both the net head of the pump turbine and loss head at the inlet valve in mind, the flow rate characteristic of the pump turbine at this time is expressed as follows: EQU H'=H-.zeta.Q.sup.2
Thus, for example, the no-load operation point C (n, q) of the curve La in FIG. 1 is shifted to a point C' (n', q') of the curve Lc. At this time, n', q' are expressed as follows: ##EQU1##
From the above it is understood that with the inlet valve set to the smaller opening the S-shaped flow rate characteristic curve of the pump turbine including the inlet valve will become gentle. Even where the guide vanes are in the inoperative state upon load rejection, if the inlet valve is closed, there is an advantage that it is possible to suppress the self-excited oscillation state due to the S-shaped flow rate characteristic of the pump turbine.